This invention relates to a method and apparatus for improving core loss by refining the magnetic domain wall spacing of electrical sheet or strip products. More particularly, this invention relates to method of processing final texture annealed grain-oriented silicon steels to permanently refine the domain structure using local hot deformation.
Grain-oriented silicon steel is conventionally used in electrical applications, such as power transformers, distribution transformers, generators, and the like. The steel's ability to permit cyclic reversals of the applied magnetic field with only limited energy loss is a most important property. Reductions of this loss, which is termed "core loss", is desirable.
In the manufacture of grain-oriented silicon steel, it is known that the Goss secondary recrystallization texture, (110)[001] in terms of Miller's indices, results in improved magnetic properties, particularly permeability and core loss over non-oriented silicon steels. The Goss texture refers to the body-centered cubic lattice comprising the grain or crystal being oriented in the cube-on-edge position. The texture or grain orientation of this type has a cube edge parallel to the rolling direction and in the plane of rolling, with the (110) plane being in the sheet plane. As is well known, steels having this orientation are characterized by a relatively high permeability in the rolling direction and a relatively low permeability in a direction at right angles thereto.
In the manufacture of grain-oriented silicon steel, typical steps include providing a melt having on the order of 2-4.5% silicon, casting the melt, hot rolling, cold rolling the steel to final gauge typically of 7 or 9 mils, and up to 14 mils with intermediate annealing when two more cold rollings are used, decarburizing the steel, applying a refractory oxide base coating, such as a magnesium oxide coating, to the steel, and final texture annealing the steel at elevated temperatures in order to produce the desired secondary recrystallization and purification treatment to remove impurities such as nitrogen and sulfur. The development of the cube-on-edge orientation is dependent upon the mechanism of secondary recrystallization wherein, during recrystallization, secondary cube-on-edge oriented grains are preferentially grown at the expense of primary grains having a different and undesirable orientation.
As used herein, "sheet" and "strip" are used interchangeably and mean the same unless otherwise specified.
It is also known that through the efforts of many prior art workers, cube-on-edge grain-oriented silicon steels generally fall into two basic categories: first, regular or conventional grain-oriented silicon steel, and second, high permeability grain-oriented silicon steel. Regular grain-oriented silicon steel is generally characterized by permeabilities of less than 1870 at 10 Oersteds. High permeability grain-oriented silicon steels are characterized by higher permeabilities which may be the result of composition changes alone or together with process changes. For example, high permeability silicon steels may contain nitrides, sulfides, and/or borides which contribute to the particles of the inhibition system which is essential to the secondary recrystallization process for the steel. Furthermore, such high permeability silicon steels generally undergo heavier cold reduction to final gauge than regular grain oriented steels; a final heavy cold reduction on the order of greater than 80% is made in order to facilitate the high permeability grain orientation. While such higher permeability materials are desirable, such materials tend to produce larger magnetic domains than conventional material. Generally, larger domains are detrimental to core loss.
It is known that one of the ways that domain size and thereby core loss values of electrical steels may be reduced is if the steel is subjected to any one of various practices designed to induce localized strains in the surface of the steel. Such practices may be generally referred to as "domain refining by scribing" and are performed after the final high temperature annealing operation. If the steel is scribed after the final texture annealing, then there is induced a localized stress state in the texture-annealed sheet so that the domain wall spacing is reduced. These disturbances typically are relatively narrow, straight line patterns, or scribes, generally spaced at regular intervals. The scribe lines are substantially transverse to the rolling direction and typically are applied to only one side of the steel.
In fabricating electrical steels into transformers, the steel inevitably suffers some deterioration in core loss quality due to cutting, bending, and construction of cores during fabrication, all of which impart undesirable stresses in the material. During fabrication incident to the production of stacked core transformers and, more particularly, power transformers in the United States, the deterioration in core loss quality due to fabrication is not so severe that a stress relief anneal (SRA), typically about 1475.degree. F. (801.degree. C.), is essential to restore properties. For such end uses there is a need for a flat, domain-refined silicon steel which need not be subjected to stress relief annealing. In other words, the scribed steel used for this purpose does not have to possess domain refinement which is heat resistant.
However, during the fabrication incident to the production of most distribution transformers in the United States, the steel strip is cut and subjected to various bending and shaping operations which produce more working stresses in the steel than in the case of power transformers. In such instances, it is necessary and conventional for manufacturers to stress relief anneal (SRA) the product to relieve such stresses. During stress relief annealing, it has been found that the beneficial effect on core loss resulting from some scribing techniques, such as mechanical and thermal scribing, are lost. For such end uses, it is required and desired that the product exhibit heat resistant domain refinement (HRDR) in order to retain the improvements in core loss values resulting from scribing.
In referring now to certain prior teaching, U.S. Pat. Nos. 4,533,409, issued Dec. 19, 1984 and U.S. Pat. No. 4,711,113, issued Dec. 8, 1987 disclose a method and apparatus for scribing a grain-oriented silicon steel to refine the grain structure by passing the cold strip through a roll pass defined by an anvil roll and scribing roll having a surface with a plurality of projections extending along the roll axis. The anvil roll is typically constructed from a material that is relatively more elastic than the material from which the scribing roll is constructed. Preferably, the scribing roll is constructed from steel and the anvil roll is constructed from rubber. The process described in U.S. Pat. No. 4,711,113, maybe performed prior to or after final texture annealing but the domain refinement achieved is not maintained through the usual stress relief annealing temperatures.
U.S. Pat. No. 4,742,706, issued May 10, 1988 discloses an apparatus for imparting strain to a moving steel sheet at linear spaced apart deformed regions. The apparatus includes a strain imparting roll having a plurality of projections as in the above described U.S. Pat. No. 4,711,113. The apparatus of the '706 patent also includes a press roll, a plurality of back-up rolls and fluid pressure cylinder interconnected so as to control pressure against the press roll.
U.S. Pat. No. 4,770,720, issued Sept. 13, 1988 discloses cold deformation technique wherein final texture annealed grain oriented silicon steel at as low as room temperature, preferably 50.degree. to 500.degree. C. (122.degree. to 932.degree. F.) is subjected to local loading, at a mean load of 90 to 220 kg/mm.sup.2 to (127,000 to 325,000 PSI) to form spaced apart grooves. The sheet must then be annealed at 750.degree. C. (1380.degree. F.) or more so that fine recrystallized grains are formed to divide the magnetic domains and improve core loss values which survive subsequent stress relief annealing.
The present invention provides a new method characterized by low cost scribing practice compatible with conventional steps and equipment for producing grain-oriented silicon steels. Furthermore, the method applies a uniform scribing operation in a continuous processing line in a relatively uncomplicated manner.